1. Field of the Invention
The invention is in the field of methods and apparatus for removing submicron particles from gaseous effluents containing sulfur dioxide and other substances in addition to submicron particles. More specifically, the present invention has particular application for the removal of such particles from flue gas produced by burning coal.
2. Related Technology
Throughout the years, a great deal of technology has been developed for removing pollutants from gaseous effluents produced by combusting fossil fuels, and manufacturing processes such as these involving manufacture of gasoline and other hydrocarbonaceous products. These pollutants have comprised a large variety of substances, among which are typically included such substances as NO, NO.sub.2, SO.sub.2, SO.sub.3, and fine particulates. These substances can be very damaging to human health if present in sufficient quantities. Unfortunately, such damaging quantities can and do exist unless adequate precautions are taken. Due to increased energy demands the number of polluting sources has increased greatly during the last few decades, and thus there is a steadily increasing need for increased precautions. This has led to the issuance of State and Federal standards and regulations, such as the Clean Air Act of 1977, the National Energy Act, the National Primary and Secondary Ambient Air Quality Standards, etc. High on the list of substances to be controlled are the fine particulates, and the NO.sub.x (NO and NO.sub.2), and the SO.sub.x (SO.sub.2, and SO.sub.3) substances. SO.sub.3 is particularly dangerous since it reacts with water to form sulfuric acid, which falls as acid rain if vented to the atmosphere. Additionally, SO.sub.3 causes rapid deterioration of equipment downstream from the combustor.
Fine particulates are dangerous to human health in that they deposit in the respiratory system and the lungs. Many of these particles may comprise toxic substances, such as lead, arsenic, antimony, bismuth, mercury, and cadmium. Fortunately, the respiratory system in most individuals can remove most particles larger than about 3 .mu.m. However, the smaller particles are capable of penetrating deep into the lungs wherein they increase airway resistance and susceptibility to infection, and reduce lung function. Unfortunately, the air quality standards, which are based merely on the mass concentration of total suspended particulate material (TSP), do not relate very well to damage to human health. Although the total mass of particulates larger than about 1 .mu.m far exceeds the mass of the smaller particles the number of submicron particles far exceeds the number of the larger particles. Additionally, the surface area of the smaller particles greatly exceeds the surface area of the larger particles, and thus the condensation of toxic elements occurs chiefly on the smaller particles. There are presently no regulations for controlling specifically the discharge of submicron particles into the atmosphere because of the lack of any significant method or system for controlling such. Consequently, such a method and system are greatly needed.
As noted above, other polluting substances which must be removed are SO.sub.2 and SO.sub.3. Typically the SO.sub.2 comprises about 98% to 99% of the total SO.sub.x, and the SO.sub.3 about 1% to 2% of the total SO.sub.x. However, the SO.sub.3 is very damaging, as noted above. If not removed or reduced during or following combustion then the temperature of the flue gas must be vented at a temperature higher than the acid dew point, about 132.2.degree. C. for a typical coal, in order to prevent equipment damage due to the formation of sulfuric acid.
Among current methods for such removal are coal cleaning processes, practiced prior to combustion of the coal. However, these methods represent a not inconsiderable expense.
Another method involves the use of flue gas desulfurizing (FGD) equipment, such as wet scrubbers, after combustion. However, this results in a significant loss of the heat of combustion of the coal. As an example, wet scrubbers typically reduce the temperature of the flue gas to the bulk water dew point, about 52.5.degree. C. for a typical flue gas. It will be assumed that in the absence of FGD equipment heat is first recovered from the flue gas so as to reduce the temperature to a value somewhat above the acid dew point, approximately 132.2.degree. C. for a typical flue gas. However, if FGD equipment is used, the temperature is reduced to about the bulk water dew point, being about 52.2.degree. C. for a typical flue gas. Reducing the temperature from the acid dew point to the bulk water dew point represents about 4.4% of the heat of combustion, which is lost. After scrubbing, the flue gas must be reheated above the acid dew point since the scrubbing process removes the SO.sub.2 but only partially removes the SO.sub.3. This represents another 4.4% of the heat of combustion. Thus, the total heat lost is 4.4+4.4=8.8% of the heat of combustion.
Another popular method involves the use of fluidized-bed combustors (FBCs). FBCs have the capability to remove much of the SO.sub.x by using absorbent materials in the bed, thus eliminating the need for coal cleaning or flue gas desulfurizing. However, the particulate emissions from FBCs are greater than those from pulverized-coal combustors (PCCs) due to the elutriation of bed material and the necessity of recycling unburned carbon used in the FBC process. Thus, when FBCs are used more efficient particle separators must be used. Electrostatic precipitators, such as are commonly used in conjunction with PCCs, do not work well with the particulates from FBCs since the dust resistivity from FBCs is much higher, being &gt;10.sup.11 .OMEGA.cm rather than the usual &lt;5.times.10.sup.9 .OMEGA.cm from PCCs. Furthermore, particles less than 0.1 to 0.2 microns do not move in response to gravitation or electrostatic forces. The standard method, at least in the United States, is to use fabric filters. In order to reduce wear and tear on these filters the flue gases exiting the usual recycle cyclones are first cooled, so as to reduce temperature and velocity, before being introduced to the filters. All of this represents increased cost. Thus, once again, a simple method and system for removing submicron particles is very much needed.
Other pollutants which must be removed are the NO.sub.x gases. Various methods are used today. One such is the selective noncatalytic NO reduction process, commonly termed the Thermal DeNO.sub.x process. This process is described in U.S. Pat. No. 3,900,554 to Lyon, issued Aug. 19, 1975, and thus is not described further herein. In this process nitric oxide (NO), the predominant NO.sub.x component in flue gas (about 90%) is reduced, at least partially, by contact with ammonia and oxygen at approximately 1000.degree. C., eventually resulting in nitrogen and water. When performed properly, and controlled closely, very little ammonia is left over. Another process is the selective catalytic NO reduction process, commonly termed the catalytic DeNO.sub.x process, which involves the processing of the gaseous effluent and ammonia over a catalyst, such as copper, at about 400.degree. C. The copper is converted to an oxide, and then to a sulfate, and then the NO.sub.x reduction begins. This process is well known in the industry and thus is not described further herein.
Still another method is to limit the excess air which is supplied to the combustor. However, all of these methods have their drawbacks.
Another aspect worthy of note relates to the recovery of sensible heat from the flue gas. Normally, since SO.sub.3 is present, heat can only be recovered to the point where the temperature of the flue gas is higher than the acid dew point, 132.2.degree. C., so as to avoid the formation of sulfuric acid, as noted above. However, if the SO.sub.3 could be removed or reduced, then sensible heat could be recovered to the point where the temperature approached the buoyancy temperature necessary for venting, about 82.2.degree. C., as noted above.
There is a process whereby SO.sub.3 can be reduced to SO.sub.2 which involves the injection of methanol into the flue gas. However, this process does not remove the SO.sub.2. It would be of great advantage if the SO.sub.2 could be removed along with removal of the submicron particulates.